GaN group compounds have direct bandgap (Eg) in the ultraviolet and Blue/Green light wave band, and hence may be used as highly efficient sources of white light and visible light. At present, commercialized products have blue, green, ultraviolet and white light-emitting diode (White LED) and laser diode (LD) in the blue and violet light wave band. The energy level of GaN group compounds have high electron affinity (χ). Take GaN for instance, Eg=3.4 eV, χ=2.9 eV. Hence when forming metal contact electrodes, especially p-type GaN based ohmic contact electrodes, metals of a high work function (φm) must be used, such as Nickel (Ni) (φm=5.15 eV), Gold (Au) (φm=5.1 eV), Palladium (Pd) (φm=5.12 eV), Platinum (Pt) (φm=5.65 eV), Ruthenium (Ru), Iridium (Ir), and the like. Hole injection is performed from the metal electrode to the p-type GaN. For instance, U.S. Pat. No. 5,563,422, assigned to Nichia Chemical Industries, Ltd., discloses a technique which uses Ni/Au/p-GaN as the contact electrode of p-GaN, and forms a transparent conductive film through an annealing process at a temperature higher than 400° C. Another technique is disclosed in U.S. Pat. No. 6,620,643 (Sep. 6, 2003), assigned to Toyoda Gosei., Ltd. It uses Au/Co/p-GaN as the contact electrode of p-GaN.
The mechanism of the high work function metal being able to form p-GaN ohmic contact after annealing is explained in an article published by H. W. Jang et al, entitled “Mechanism for ohmic contact formation of oxidized Ni/Au on p-type GaN,” Journal of Applied Physics vol. 94, No. 3, pp. 1748–1752 (Aug. 1, 2003). In the oxidation process at high temperature, take an Au/Ni/GaN interface as an example, the high temperature causes Ni to be diffused and form a p-type transparent conductive film NiO/Au/GaN on the surface of GaN. On the interface, not only Ga is melted in Au (for example, at 409° C. atoms of the low melting point Ga has a meltability of 13% in Au). The remained atoms of Ga on the interface state still have reactions with oxygen to form Gallium Oxide (Ga2O3). It is further understood that the atoms of Ga on the interface are melted and oxidized to form changes of the interface state. According to the research done by J. L. Lee et al (Applied Physics Letters vol. 74, 2289 (1999)). (1) It is not only Ga vacancy formed on the interface, that is equivalent to forming acceptors on the p-GaN interface, thus providing extra electron hole concentration on the p-GaN surface. Meanwhile, according to the research done by A. Motayed et al. “High-transparency Ni/Au bilayer contacts to n-type GaN,” Journal of applied physics vol. 92, no. 9, pp. 5218–5227 (Nov. 1, 2002). (2) The surface work function (φm) may also decrease. Based on the reasons (1) and (2) set forth above, with the GaN surface condition changed, Fermi level on the GaN surface may be improved to facilitate forming of ohmic contact electrodes with NiO.
However, research done by H. W. Jang et al. “Transparent ohmic contacts of oxidized Ru and Ir on p-type GaN,” Journal of applied physics vol. 93, no. 9, pp. 5416–5418 (May 1, 2003) indicates that the thermal stability of the electrode formed by Ni/Au/GaN is not desirable. For instance, after having been annealed at 500° C. for 24 hours, the resistance of Ni/Au increases 840 times. Moreover, Au has strong absorption at the visible wave band. For fabricating Blue/Green light LEDs, a super thin Au film (5–10 nm) has to be used for the transparent conductive layer. Hence the present technology development of p-GaN ohmic contact electrodes mainly focuses on non-Au electrodes.
Recently published techniques include:                (1) n:ITO/Au/Ni/p-GaN (S. Y. Kim et al., “Effect of an indium-tin-oxide overlayer on transparent Ni/Au ohmic contact on p-type GaN,” Applied Physics Letters, vol. 82, No. 1, pp. 61–63, (Jan. 6, 2003), and        (2) Al:ZnO/Ni/p-GaN (J. O. Song et al., “Highly low resistance and transparent Ni/ZnO ohmic contacts to p-type GaN,” Applied Physics Letters vol. 83, No. 3, pp. 479–481 (Jul. 21, 2003).        
The techniques set forth above use respectively the following transparent oxidized films that are stacked to form ohmic contact electrodes with p-GaN: ITO (60 nm)/Au (3 nm)/Ni (2 nm)/p-GaN (annealed at 500° C.); and AZO (450 nm)/Ni (5 nm)/p-GaN (annealed at 550° C.).
The two techniques mentioned above have commonly adopted a n-type transparent conductive oxide film of a high work function (φm>4 eV), such as Sn:In2O3 (ITO), Al:ZnO (AZO), and is stacked top to tail on top of each other with a p-type transparent conductive oxide film NiO of a proximate work function (φm=5 eV) to form an ohmic contact with a semiconductor (such as GaN) which has high electron affinity. This is a work function engineering concept. Namely, use two n/p-type transparent oxide films of proximate work function to couple with a wide bandgap material (such as GaN) to form an ohmic contact.
The p-GaN electrode is made by Indium Tin Oxide (ITO) and Zinc Oxide which adopts aluminum (AZO) are stacking with the Nickel Oxide (NiO) has two limitation:                (1) The difference of work function among ITO, AZO and NiO ranges from 0.3 to 0.5 eV.        (2) The absorption bandgap values of ITO, AZO and NiO are about 3.6 eV, which is proximate to GaN.        
The first limitation (1) will boost the forward voltage of the GaN LED by 1˜5 V. The second limitation (2) will result in a lower penetration rate of the ultra-violet light when those two oxide films are stacked. As a result, the external efficiency of the short wavelength drops.
For instance, a:ITO(250 nm)/Ni(10 nm)/p-GaN and b:Au (5 nm)/Ni(10 nm) will have the penetration rate dropping to 55% at 350 nm∘(Reference for a can be found in: R.-H. Horng et al., “Low-resistance and high-transparency Ni/ITO ohmic contacts to p-type GaN,” Applied Physics Letters, vol. 79, no. 18, pp. 2925–2927 (Oct. 29, 2001)∘Reference for b can be found in: J. K. Ho et al., “Low resistance ohmic contact to p-type GaN,” Applied Physics Letters vol. 74, no. 9, pp. 1275–1277 (Mar. 1, 1999).